This invention relates to orthopedic surgery, more particularly to a prosthetic nuclear replacement for a damaged intervertebral disc, and a surgical procedure for implanting the construct in the intervertebral disc space.
The normal intervertebral disc has an outer fibrous ring, constituted mainly of collagen fibers, which strongly binds the vertebral elements together. This fibrous outer layer, or annulus, encircles a soft gel-like matrix, or nucleus, which serves both as a cushion and as a mobile and compressible element that allows motion to occur between the vertebral bodies above and below the intervertebral disc. This gel matrix is 95% water. The types of motion that can occur at the level of the intervertebral disc include flexion, extension, lateral bending and varying degrees of torsion or rotation.
In the course of a day, the normal intervertebral disc may encounter various combinations of these bending or twisting motions several thousand times. As a consequence of such repetitive motion, natural discs deteriorate over time, much as the padded cushion on a well-used chair might do.
The effect of this deterioration is a loss of water content of the gel matrix of the nucleus and a concomitant compacting of its fibers with a resultant loss of disc space height which in turn causes a loosening of the surrounding support ligaments of the spine and the development of what is termed degenerative instability. This instability results in a pathologic excess of movement at the intervertebral disc space that further accentuates the degeneration of both the nucleus and the annulus of the disc. With continued deterioration, the annulus of the disc can bulge or even develop radial tears that allow the inner nuclear material to protrude or even extrude from the disc space. This bulging of the annulus or protrusion of the nucleus can compress nerves and cause disabling sciatic pain. Distension or bulging of the annulus alone is frequently sufficient to produce disabling back pain because of compression or inflammation of free nerve endings present in the outer annulus of the disc.
The time-honored method of addressing degenerative lumbar instability resulting from severely damaged intervertebral discs has been to remove the damaged disc completely and fuse the two adjacent vertebral bones to eliminate pathological motion. While the approach does well at eliminating pathological motion, it also prevents any natural motion at that segment. The consequence of eliminating natural motion at a single segment generally is that greater degrees of stress occur above or below that segment. This in turn accelerates degeneration of the neighboring intervertebral spaces, often necessitating additional fusion surgeries.
It would be desirable, therefore, to preserve natural motion at every disc space and thus eliminate the degenerative domino effect that discectomy and fusion seems to produce. Since the earliest pathologic change evident in a degenerative disc is loss of water content with concomitant loss of disc space height, maintenance of disc space height seems critical for maintaining the way opposing vertebral surfaces alter position with each other during bending and twisting. Indeed, loss of disc space height seems to be the more crucial early feature of degenerative instability. With degenerative instability the ligaments may ultimately become so lax that buckling of the ligaments occurs, or even pathologic slippage of the spine (spondylolithesis) may result. Preserving disc space height is therefore important in preventing secondary degenerative changes that occur as a consequence of loss of the disc space height from mechanical damage or dessication due to aging.
An intervertebral disc nuclear prosthesis ought, ideally, to restore and preserve disc space height while permitting sufficient natural motion (flexion, extension, lateral bending and rotation) to prevent excessive stresses on spinal segments above and below the prosthesis. Natural motion may also play a role in the health of the annulus and surrounding ligaments, much as natural stresses play a role in the maintenance of strength and density of normal living bone.
Many synthetic structures have been used as intervertebral disc implants, but few materials are durable enough to withstand the tremendous and repetitive forces a natural disc must withstand. In addition, the majority of intervertebral implants fail to restore and maintain sufficient disc space height to keep spinal support ligaments taut. Maintenance of a physiologic degree of tautness seems crucial to the long-term viability of spinal support ligaments.
Kuslich, in U.S. Pat. No. 5,571,189, describes an expandable porous fabric implant designed to stabilize a spinal segment. The fabric is porous and packed with biologic material which favors fusion of the interspace rather than functional mobility. It is packed with material which stabilizes a spinal segment by allowing ingrowth of bone and fibrous issues through pores on its surface.
In U.S. Pat. No. 5,674,295, Ray describes a pillow-shaped prosthetic spinal disc nucleus body made of a hydrogel core and a flexible constraining jacket which permits the hydrogel core to expand and contract. The constraining jacket is also porous, allowing entry and egress of fluids. The jacket is not filled with a hardenable material, since significant expansion and contraction is a pivotal feature to the biologic function of the prosthesis he describes.
U.S. Pat. No. 3,875,595 to Froning describes a bladder-like prosthesis which is inflated with liquid or plastic, but which requires a valve as the material is non-hardenable and could potentially leak from the retaining bladder, causing the prosthesis to collapse.